Seismic refraction measurements show that the earth's crust thins from about 35 km. under the continents to 6 km. under the oceans, also that the oceanic crust has a layered structure. The sediments that cover it are much thinner than expected, which suggests that the oceans are perhaps younger than was thought, or perhaps the sediments have become engulfed in lavas. Great fracture zones cross the floor of the north-east Pacific and there is evidence for strike-slip movements of more than 200 km. along them. The discovery that heat flow rates for continents and oceans are approximately equal upsets previous ideas about radioactive heat sources. A pattern has been observed in the heat flow associated with an oceanic rise. It has been suggested that both are related to convection in the mantle. Subcrustal drag by convection currents may explain the fracture zones.